Migration Assistant? Will it move everything?

I'm running off of an external drive right now and want to switch back to using my internal drive (without re-installing), will Migration Assistant make that partition with a fresh copy of Snow Leopard installed exactly like the one I'm on now? Or will it just shift some files over? Can anyone explain a little more how it works? Thanks.

I've used migration assistant to go from one machine to another using firewire. The older machine was in target disk mode. Everything came over. I've used migration assistant to bring everything back from a Time Machine backup on a local firewire drive to the new 500GB drive I had just bought. Everything came back but there was a serious glitch with mail.app. I wound up deleting my mail and starting over. No biggie as everything was on the various servers and came back in a short period of time.

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